Overhead doors provide convenient and effective closures for large entrance openings such as entranceways to residential garages. One conventional sectional overhead door assembly 13 is shown in FIG. 1. A typical modern overhead garage door 13 includes a plurality of horizontal door sections 12 pivotally connected together by hinges 15. Rollers 14 on ends of the door sections 12 ride in pairs of roller tracks or rails 40, 42. Each roller track 40, 42 typically includes a vertical portion 44, 46, a horizontal portion 48, 50, and a curved transition portion 52, 54 connecting the vertical 44, 46 and horizontal portions 48, 50. The vertical portions 44, 46 of these roller tracks 40, 42 are mounted along opposed vertical edges of the garage entranceways 60. At or near the top of the entranceway 60, the curved portions 52, 54 of the tracks 40, 42 curve inwardly into the enclosed space. Substantially horizontal portions 48, 50 of the tracks 40, 42 extend into the garage at or near the elevation of the top of the entranceway 60. In a closed position like that shown in FIG. 1, the rollers 14 on the door sections 12 are supported in the vertical portions of the roller tracks, thereby supporting the door such that the door spans and covers the entranceway 60. A latching/locking mechanism may secure the door 13 against upward movement (not shown).
A typical sectional overhead door 13 is opened by raising the door sections 12 along the roller tracks 40, 42. As the door sections 12 are raised, the rollers 14 travel along the vertical track portions 44, 46, enter and travel along the curved track portions 52, 54, and finally enter and travel along the horizontal track portions 48, 50. Accordingly, the pivotally-connected door sections 12 are moved from a vertical, closed orientation, to a substantially horizontal, open orientation. When fully open, the door is positioned entirely within the garage space at or above the topmost elevation of the doorway 60. To close the door 13, the door sections 12 are guided by the tracks 40, 42 to a closed, vertical orientation.
As a sectional overhead door is lifted and the rollers enter the horizontal portion of the roller tracks, the weight of the door progressively is carried by the horizontal portions of the tracks. For example, for a five-panel sectional overhead door like that shown in FIG. 1, once the rollers 14 on the uppermost door panel 12 engage the horizontal portions 48, 50 of the roller tracks 40, 42, the weight of the engaged uppermost panel 12 is supported by the tracks 40, 50. Accordingly, the total force required to further lift the other four door panels 12 is about 80 percent of the total weight of the door 13. Once the door 13 is fully open, substantially the entire weight of the door 13 is carried by the tracks 40, 50.
The externally applied upward force “F” required to incrementally lift a typical sectional overhead door is at or near a maximum force “Fmax” when the door is in a closed, fully downward position. In this fully downward position, the elevation “h” of the bottom edge of the door above the floor is at a minimum elevation “hmin.” Conversely, the applied force F required to incrementally lift a typical sectional overhead door is at a minimum force “Fmin” as the door approaches its open, fully upward position. In this fully upward position, the elevation “h” of the bottom edge of the door above floor level is at a maximum elevation “hmax.” Because conventional sectional overhead doors have substantially uniform cross-sections, the incremental applied lift force F required to lift such a door is substantially inversely linearly proportional to the elevation “h” of the bottom edge of the door above floor level. Accordingly, the applied lift force F required to incrementally lift a sectional overhead door having a maximum incremental lift force Fmax and minimum incremental lift force Fmin between a fully down position (h=hmin=0) and a fully up position (h=hmax) can be expressed as:F=h[−(Fmax−Fmin)/hmax)]+Fmax A typical inversely linear relationship between the required upward lift force F and the instantaneous door elevation “h” for a conventional sectional overhead door is graphically depicted in FIG. 2.
Sectional overhead door panels typically are constructed of durable materials such as steel, wood, or the like. Accordingly, multi-panel overhead doors that include such panels are heavy to lift. For example, a typical sectional overhead door that is substantially constructed of steel sheet material may weigh 100 pounds or more. Without mechanical assistance, a single person may have difficulty or may be unable to manually lift such a door. Therefore, it is common to provide overhead door lifting systems that mechanically apply lifting forces to the doors such that the weights of the doors are substantially counterbalanced by the lifting systems. By applying counterbalancing lift forces that are slightly less than the free hanging weights of the doors (for example, 5-10 pounds less), the lifting systems permit the doors to be manually lifted with only minimal additional applied lifting force. Accordingly, such lift-assisted doors can be easily raised by a single person or by a conventional automatic door opener.
A typical sectional overhead door lifting system 10 is shown in FIG. 1. The lifting system 10 includes a torsion rod 34 rotatably mounted above the entranceway 60 and the sectional overhead door 13 by one or more mounting brackets 23, 27, 30. One or more coil torsion springs 36 are positioned on the torsion rod 34, and are fixed at one end to the torsion rod 34 by winding cones 60, and at an opposite end to a fixed bracket 30 by anchor cones 11. A cable drum 24, 26 is affixed to each end of the torsion rod 34. A cable 20 is wound on each cable drum 24, 26. Lower ends of the cables 20 are affixed to the lower end of the door 13 by cable brackets 22. When the door 13 is in a fully upward, open position, the coil torsion springs 50 are substantially unwound and apply substantially zero torsional load to the torsion rod 34 and cable drums 24, 26. Accordingly, in this fully upward, open position, the tensile load on the lift cables 20 is substantially zero. As the door 13 is lowered, however, the cables 20 are partially unwound from the cable drums 24, 26, thereby causing the drums 24, 26 and torsion rod 34 to rotate relative to the fixed bracket 30. As the torsion rod 34 rotates, the torsion spring 36 is wound and tightens, thereby creating tension in the cables 20. Accordingly, the tension in the cables 20 increases as the door 13 is lowered, thereby applying an increasing upward force to the door 13. The maximum torsional load in the torsion spring 36, maximum tension in the cables 20, and maximum resultant upward-acting force on the door 13 occurs when the door 13 is in a closed, fully downward position. Conversely, the minimum torsional load in the torsion spring 36, minimum tension in the cables 20, and minimum resultant upward-acting force on the door 13 occurs when the door 13 is in a closed, fully downward position. Typically, such torsion springs 36 have substantially linear spring constants.
In such conventional door lift systems 10, the cable drums 24, 26 have a substantially constant diameter. As shown in FIG. 3, a typical cable drum 24, 26 includes a drum cylinder 80 having a spiral cable groove 82 therearound. The cable groove 82 has a substantially constant minor radius “rm”. The term “minor radius” as used herein is the radial distance from the cable drum centerline to the root of the spiral groove at a point along the length of the spiral groove. As the a door is raised or lowered, a cable 20 is wound onto or unwound from the cable groove 82. In a conventional overhead door lift system 10, the constant-diameter drum 24, 26 and coil torsion spring 36 cooperate to provide tension in the cables 20 that is substantially inversely linearly proportional to the elevation of the bottom of the door 13 from the floor. Desirably, the lift system 10 is configured such that the upward lift force applied to the door 13 by the cables 20 is only slightly less than the free-hanging weight of the door 13 at any point along the upward or downward movement of the door 13. Accordingly, the linearly variable upward lift force applied by the lift system 10 is effective to substantially counterbalance a majority of the downwardly acting, substantially linearly variable weight of the door 13 (like that depicted in FIG. 2). A properly designed lift system 10 permits a sectional overhead door 13 to be easily lifted by applying a substantially constant, upward manual force of only about 5-10 pounds, and more preferably about 5 pounds. Such a properly balanced system 10 prevents a rising door 13 from surging upward due to excessive counterbalancing lift forces, permits a door 13 to be slightly biased toward a closed position by the door's own weight, and prevents a closing door 13 from slamming shut due to insufficient counterbalancing lift forces. Such a system 10 also permits a typical sectional door 13 to be operated by a conventional automatic door opener.
Because upward-acting sectional overhead doors have large surface areas, such doors can be vulnerable to high static or dynamic wind loads. Many modern international, state, and local building codes currently require upward-acting doors such as residential overhead garage doors to be capable of withstanding high transverse wind loadings such as those that may be experienced during hurricanes or other storms. For example, many building codes in wind-prone locales require overhead garage doors in newly constructed residential structures to comply with wind-load testing standards such as ASTM E 330-97. In order to comply with such regulations, overhead garage door manufacturers have developed reinforcement systems to bolster the strength and stiffness of sheet metal overhead garage doors. As shown in FIG. 4, such reinforcement systems may include reinforcement struts 102 horizontally mounted across the back sides of the thin-walled door panels 104 of a sectional overhead door 100 with suitable fasteners 106 or the like. Such reinforcement struts 102 may be channels, tubes, T-shaped members, U-shaped members, or any other strut configuration having sufficient strength and stiffness.
Though such reinforcement struts 102 have proven to be highly effective in strengthening overhead doors 100 to withstand high transverse wind loads, these struts 102 also change the substantially linear weight-to-height door characteristic described above for doors with substantially uniform cross-sections and without reinforcement struts. A representation of a weight-to-height characteristic for a typical strut-reinforced door 100 is shown in FIG. 4. As can be seen by comparing FIG. 4 to FIG. 2, the addition of reinforcement struts 102 to a sectional overhead door 100 causes the weight-to-height characteristic of the door 100 to change from a substantially linear relationship to a distinctly non-linear relationship. This change primarily is attributable to the non-linear weight distribution between the top and bottom of the strut-reinforced door 100.
Conventional sectional overhead garage doors typically include four or more pivotally connected door sections. Some newer sectional overhead door designs, however, may include only three door sections, such as the overhead doors described in co-pending U.S. patent application Ser. No. 10/699,749, filed Nov. 3, 2003. Such three-section doors tend to have a height-to-weight relationship that is substantially non-linear when compared to the substantially linear height-to-weight relationship for four-panel doors. Especially when such three-panel doors are provided with one or more reinforcement struts like those described above, a substantially nonlinear door height/weight relationship like that shown in FIG. 5 results.
Known substantially linear garage door lift systems like those described above are incapable of providing variable upward-acting forces to adequately counterbalance the non-linear change in door weight associated with changes in door elevation for wind-resistant, strut-reinforced doors overhead doors and/or doors comprised of three sections. Accordingly, there is a need for a door lift system that accommodates this non-linear variability in door lift forces such that a substantially constant applied vertical load is sufficient to raise such a door.